Circularity potential
Ultra low
Strength
Medium
Production energy
High
Stiffness
Low
Embodied CO2
Medium
Density
Medium

Unsaturated polyester resin (UPR) is a relatively low cost and durable thermosetting plastic made up of two parts: liquid resin and catalyst (hardener). The principal difference between these systems and thermoplastic polyesters, such as polyethylene terephthalate (PET), is the inclusion of double bonds in the polymer chain, which allows for permanent cross-linking and the formation of a rigid thermoset structure. This has many advantages: the resin starts off in liquid state and so suitable for casting, pouring, coating, composite laminating and so on; very good resistance to chemicals and weathering; and high adhesive strength with excellent wetting (ability to coat a solid surface, like glass fibre). However, on is own it is quite brittle and so tends to be used in combination with fibre or mineral reinforcement. Also, as a thermoset, it is not practical to recycle. This problem is especially acute in the production of wind turbine blades, where thousands of tons of this material are being consumed each year and will inevitably end up in landfill.

Unsaturated polyester resin contains some harmful chemistry, such as individual ingredients like styrene, a known carcinogen, as well as catalysts like methyl ethyl ketone peroxide (MEKP) that can cause skin damage and blindness.

There are two main families: ortho (based on phthalic anhydride) and iso (based on isophthalic acid). Ortho types are the most common and used in everything from boat hulls to bathtubs and countertops. Iso types are higher priced and higher performance. They are used in applications that require higher levels of scratch resistance, clarity and weather resistance. For example, iso resins are used in gel coats, which provide the outer decorative and protective layer on FRP structures. It is painted onto the mould before the fibreglass is applied behind. So when demoulded, it becomes the visible layer.


Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Low circularity potential
Potentially toxic in use
Hazardous end of life
Microplastics


Polyester powder coating is used widely in construction, architecture, industrial, transportation, marine and furniture applications. For example, metalwork (steel and aluminium) on car exteriors like wheels and trim; and buildings, like gutters, roofs and cladding, is coated with a colourful polyester around 60-80 microns thick. The polyester is applied in solid, powder form, usually by electrostatic spraying (drawing the powder to the surface to reduce over-spray). It is cured and fused with heat to produce a high performance finish. It is cost effective and durable, making up more than half of the powder coating market, but not particularly sustainable. Fillers and additives are added at ratios of up to 50% to further reduce cost.

The carbon footprint is 3-5.5 kgCO2e/kg, which is equivalent to around 0.3-0.45 kgCO2e/sqm. For reference, solvent based coatings give around twice the emissions.

There are several options with the resin chemistry, which allows the coating to be tailored to an application – in terms of flexibility, toughness, chemical and corrosion resistance – both for interior and exterior applications. It is available in matte, satin and gloss finishes. However, in exterior applications, the gloss can dull.

Hybrid coatings are a mix of polyester with epoxy or acrylic. They are most commonly used for furniture and interior applications.

Polyurethane coatings are a polyester-isocyanate. They are known for durable in exterior environments. They exhibit superior chemical resistance compared to standard polyesters and are known for exterior durability. Anti-graffiti coatings are often polyurethane. However, they give off more volatile organic compounds (VOC) than regular polyester, making them more harmful in production and use.


Design properties
Cost usd/kg
2-3
Embodied energy MJ/kg
63-110
Carbon footprint kgCO2e/kg
3-5.5
Density kg/m3
1100-1430
Tensile modulus GPa
1-3.3
Tensile strength MPa
33-63
Hardness Mohs
2.5
Thermal expansion (µm/m)/ºC
100-200
Heat deflection temperature ºC
85
Thermal conductivity W/mK
0.21
Temperature min-max °C
-40 to 100
Thermal
insulator
Electrical
insulator